Radars without moving target indication use a directional antenna and transmit pulses when scanning the horizon. The time from transmission to reception of a pulse provides the range to a reflecting target. By drawing the received signal power versus antenna angle, the resulting image is a representation of the surrounding environment. Navigation radars use this principle to draw land area and other ships on the screen.
FIG. 1 shows a conventional navigation radar system as used on most ships. This system employs a magnetron 12 as the transmitter source. A magnetron is a self-oscillating tube that may deliver a strong signal at a low cost. This is a random phase device, as the heating of the tube during the transmitting pulse will change its resonant frequency and shift the phase of the emitted signal. The signal from the magnetron 12 is lead to a scanning antenna 11. The corresponding reflected or back-scattered signals are received by the antenna 11 and processed in a receiving channel including the components RF amplifier 14, mixer 15, local oscillator 18, Intermediate Frequency amplifier and filter 16, and detector 17. The detector 17 is normally a crystal (diode) amplitude detector. The signals from the detector 17 may be observed on an old fashioned Plan Projection Indicator (not shown), or digitized and observed on a raster display device (computer screen).
To protect the receiving channel from the strong transmitter signal, the signals are routed through a circulator 13. In addition, the receiving channel will be muted or turned off during the transmitting period.
In order to detect targets in a cluttered environment (e.g. over land), a radar has to include means for extracting moving targets from the surrounding clutter. Multiple methods have been used, but may be divided into two main categories:                Non-coherent detection where the detected video (amplitude) signal from two or more antenna scans are compared in amplitude to detect changes, thus not utilising signal phase        Coherent detection where signal phase changes from pulse to pulse is used to extract        
The system illustrated in FIG. 1 may to be used for detecting moving targets, i.e. according to the first alternative, if received signals are stored during a receiving period (scan), and compared with signals received during the subsequent receiving period.
Non coherent moving target indication (MTI) typically is slow (as at least two complete scans of the antenna is needed for detection) and requires that the target moves in the order of one or more resolution cell within the antenna scan time. The sensitivity is also quite low due to the fact that only detected video is used.
Most modern radar systems utilize coherent detection where the signal phase from two or more pulses is compared for detection of target radial speed towards or away from the radar. These systems employ a stable oscillator that is used both for generation of the transmitter carrier frequency and for receiver down conversion. Thus the intermediate frequency signal in the receiver is coherent with the transmitted pulse signal and the received signal phase depends on the target distance. A movement of the target towards or away from the radar corresponding to a fraction of a wavelength will result in a change in the received signal phase.
FIG. 2 shows the elements of such a coherent radar system. Here radar pulses are produced in a signal generator source 28. The generated pulses are modulating the signal from a stable carrier oscillator 212 in a mixer 29. The transmitter pulses are then amplified in a power amplifier 210 filtered in a filter 211 and delivered to the antenna 21 via the circulator 23. On receive the signals are down-converted in the mixer 25 using the carrier oscillator 212. In this way received signals will be coherent with the transmitted pulses. Subsequent pulses are compared after detection in detector 27. This system also includes an RF amplifier 24 following the circulator 23, a mixer 25 and an IF amplifier/filter 26. A moving target will show up as a phase difference between subsequent pulses. Movements as small as a fraction of the carrier wavelength may be detected.
Construction of coherent radars is costly due to the need of phase stability in high power components. Therefore a scheme of coherent-on-receive MTI has previously been employed, especially in high power radars. In this system a fast local oscillator is locked to transmitter phase and frequency during the short period of transmission. After the transmitter pulse, the oscillator system is designed to hold the phase constant in the subsequent receive period. This oscillator is used for down-conversion of the received signal, and thus provides coherent detection even if the transmitter has random phase.
FIG. 3 illustrates this concept in more detail. This system uses a free-running transmitter source 313. Phase coherence is established during receive, as the receiving channel uses a local oscillator 318 that is synchronized with the phase of the transmitting source 313. Also this system includes an RF amplifier 34, mixer 35 and IF filter/amplifier 36.
Coherent-on-receive systems require a phase locking mechanism that is fast enough to lock within the duration of a pulse and is stable enough to provide correct phase in the entire reception period. This is usually done by means of a quite complex phase lock mechanism. The system also requires a stable phase within the pulse, thus increasing the requirements to the expensive transmitter subsystem.
As explained above, existing coherent radar systems are very expensive restricting their use to e.g. surveillance of the airspace around airports. However, coherent radar systems could be very useful in other fields as well, and thus, there is a need for a radar system with similar properties, but at a lower cost.